Radio Button Value

In the site/index.php, have the following code.

 <?php
        echo CHtml::radioButtonList('choice','',array('User1'=>'User1','User2'=>'User2',), array( 'separator' => "<br>")); 
    ?>
 <?php echo CHtml::button('Submit', array('submit' => array('site/register'), )); ?>

I need to pass the selected radio button value to the controller action. How it will pass to controller action and how it access in controller action?

thanks